If it's a file you want to attach, click on the "File Manager" link below the field in which you type your message - where it says, "Post Options". Note the size of file that is permissible.
If you want to add an image from a website (Photobucket, etc.), click on the image icon (fourth from the left above the message field), and it will direct you to the place to link the image.
Hope this helps...
Mickey